SELECTION, DESIGN AND CONSTRUCTION OF
TANK SYSTEMS FOR REFRIGERATED
LIQUIFIED GAS STORAGE ON LAND
API Standard 625
First Edition, (Date Later)
Page 1 of 55
CONTENTS
1.
SCOPE ....................................................................................................................
1.1.
General.........................................................................................................
1.2.
Coverage .........................................................................................................
1.3.
Configurations .................................................................................................
1.4.
Metallic Containers ..........................................................................................
1.5.
Concrete Containers ........................................................................................
1.6.
Boundaries ......................................................................................................
1.7.
Certification .....................................................................................................
2.
REFERENCES (currently not included) .......................................................................
3.
DEFINITIONS .............................................................................................................
3.1.
General............................................................................................................
3.2.
Definitions........................................................................................................
4.
RESPONSIBILITIES ...................................................................................................
4.1.
General............................................................................................................
4.2.
Design Information ..........................................................................................
5.
SELECTION OF STORAGE CONCEPT .....................................................................
5.1.
Single Containment .........................................................................................
5.2.
Double Containment ........................................................................................
5.3.
Full Containment .............................................................................................
5.4.
Guidance on Selection of Storage Concept .....................................................
6.
DESIGN CONSIDERATIONS .....................................................................................
6.1.
Spacing Requirements ....................................................................................
6.2.
Maximum Design Liquid Level .........................................................................
6.3.
Performance Criteria........................................................................................
6.4.
Design Loads and Load Combinations ............................................................
6.5.
Seismic Analysis ..............................................................................................
6.6.
Foundation Design ..........................................................................................
6.7.
Thermal Corner Protection Systems (TCP for Concrete Tanks ........................
7.
ACCESSORIES AND APPURTENANCES .................................................................
7.1.
Access .............................................................................................................
7.2.
Process Piping.................................................................................................
7.3.
Relief Valves ...................................................................................................
7.4.
Instrumentation ................................................................................................
7.5.
Foundation Accessories ..................................................................................
7.6.
Fire and Gas ....................................................................................................
7.7.
Electrical ..........................................................................................................
7.8.
Cryogenic Liquid Spill Protection .....................................................................
7.9.
Miscellaneous ..................................................................................................
8.
QUALITY ASSURANCE AND QUALITY CONTROL ...................................................
8.1.
Introduction......................................................................................................
8.2.
NDE, Testing and Tolerances ..........................................................................
Page 2 of 55
9.
INSULATION ..............................................................................................................
9.1.
System Design ................................................................................................
9.2.
Materials ..........................................................................................................
9.3.
Load Bearing Bottom and Thermal Corner Protection (TCP) Insulation ...........
9.4.
External Wall and Roof Insulation ....................................................................
9.5.
Internal Wall Insulation ....................................................................................
9.6.
Suspended Deck Insulation .............................................................................
9.7.
Penetration and Internal Piping Insulation........................................................
9.8.
Specification for Insulation ...............................................................................
10.
POST CONSTRUCTION ACTIVITIES ........................................................................
10.1. Scope ..............................................................................................................
10.2. General............................................................................................................
10.3. Hydrostatic and Pneumatic Testing .................................................................
10.4. Drying / Purging
10.5. Cool Down
APPENDIX A Properties of Gasses .......................................................................................
APPENDIX B Foundation Settlement.....................................................................................
Figures
5-1 Single Containment Tank System Single Wall with Steel Roof and External Insulation
5-2 Single Containment Tank System Single Wall with Steel Roof, Suspended Insulation
deck and External Wall Insulation.............................................................................
5-3 Single Containment Tank System Double Wall with Steel Inner Tank, Steel Outer Tank
and Steel Roof .........................................................................................................
5-4 Single Containment Tank System Double Wall with Steel Inner Tank, Steel Outer Tank
and Double Steel Roof .............................................................................................
5-5 Double Containment Tank System Double Wall with Steel Inner Tank, Steel Outer
Containment and Steel Roof ....................................................................................
5-6 Double Containment Tank System Double Wall with Steel Inner Tank, Concrete Outer
Containment and Steel Roof ....................................................................................
5-7 Full Containment Tank System Double Wall with Steel Inner Tank, Steel Outer
Containment and Steel Roof ....................................................................................
5-8 Full Containment Tank System Double Wall with Steel Inner Tank, Concrete Outer
Containment and Steel Roof ....................................................................................
5-9 Full Containment Tank System Double Wall with Steel Inner Tank, Concrete Outer
Containment and Concrete Roof ..............................................................................
6-1 Maximum Design Liquid Level
Tables
10-1 Recommended Drying and Nitrogen Purging End Points for Steel Tanks
A-1 Physical Properties of Gasses
Page 3 of 55
Draft for API Refrigerated Tank Task Group
SECTION 1 - Scope
Draft No.
G
Date:
October 30, 2007
Drafted by:
Larry Hiner and Doug Miller
Chicago Bridge and Iron Company
14105 S. Route 59
Plainfield, IL 60544-8984
Telephone: 815 439-6125
Fax: 815 439-6654
Email: LHiner@CBI.com, DMiller@CBI.com
Writers Comments:
Changes since October 1, 2007 draft are in blue.
1.1
General
This standard is for low pressure, aboveground, vertical, cylindrical, tank systems
storing liquefied gases requiring refrigeration. This standard provides general
requirements and guidance on selection of storage configuration, materials, design,
construction, testing, and commissioning of tanks and their associated systems
including insulation, foundations and appurtenances. These general requirements
address issues common to all of these tank systems, issues involving coordination of
the components of the tank system, and issues of the tank system acting in an
integrated way. However, the detailed requirements applicable to the metallic and
concrete containers respectively are contained in the standards named in 1.4 and
1.5. It is a mandatory requirement of this standard that the applicable portions of the
named standards are satisfied.
1.2
Coverage
This standard covers tank systems having storage capacity in excess of 800 cubic
meters (5000 bbls) storing liquids which boil at ambient temperature and pressure
and require refrigeration to less than 5oC (40oF) to maintain a liquid phase. The
minimum design temperature is 198oC (-325oF), the maximum design internal
pressure is 50 kPa (7 psig), and the maximum design uniform external pressure is
1.75 kPa (0.25 psig).
1.3
Configuration
Various configurations are addressed in Section 5.0. All configurations consist of a
primary containment constructed of metal, concrete, or a metal / concrete
combination and when required a secondary containment constructed of metal,
concrete, or a metal /concrete combination.
Page 4 of 55
1.4
Metallic Containers
Metallic container materials, design, fabrication, inspection, examination, and testing
shall be in accordance with API 620 including either Appendix R or Appendix Q. The
applicable appendix of API 620 depends on the design metal temperature and the
applicable temperature ranges given in these appendices.
1.5
Concrete Containers
Concrete container materials, design, construction, inspection, examination, and
testing shall be in accordance with ACI 376. Metallic components that are an integral
part of concrete contains are addressed by ACI 376 (e.g. prestressing/reinforcing
steel and metallic liners of concrete walls & roofs)
1.6
Boundaries
This standard applies to components attached to and located within the wall of the
tank system. Piping connected externally to the wall of a tank system within the
following limits shall be constructed according to this standard:
a. The face of the first flange in bolted flanged connections.
b. The first threaded joint on the pipe outside the tank wall in threaded pipe
connections.
c. The first circumferential joint in welding-end pipe connections that do not have
a flange located near the tank. (All nozzles larger than NPS 2 that are
connected to external piping shall extend outside the tank wall a minimum
distance of 8 in. and shall terminate in a bolting flange.)
1.7
Certification
The Contractor of the tank system, that will bear the API Std 625 nameplate, shall
certify that the tank system is constructed in accordance with this standard.
Page 5 of 55
Draft for API Refrigerated Tank Task Group
SECTION 3 - Definitions
Draft No.
C
Date:
January 23, 2008
Drafted by:
Anant Thirunarayanan
Bechtel
Telephone: (713)235-4544
Fax: (713)235-1650
Email: tathirun@bechtel.com
TG Chairman Comments:
Section 3, rev A was balloted at May 2007.
Red text are rev B changes in Sept 2007.
Blue text are changes after Sept 2007.
3.1
General
The definitions contained in this chapter shall apply to the terms used in this standard.
Where terms are not defined in this chapter or within another chapter, they shall be
defined using their ordinarily accepted meanings within the context in which they are
used.
3.2
Definitions
3.2.1
Refrigerated Tank System
The combination of a primary liquid container, together with secondary liquid
container (if any), insulation, containment for vapor, appurtenances,
instrumentation, and all other elements within the scope of this standard.
3.2.2
Single Containment Tank System
A single-wall tank or a tank system comprised of an inner and outer container,
designed and constructed so that only the inner container is required to
contain the product. The outer container, if any, is primarily for the retention
and protection of the insulation system and may hold the product vapor
pressure, but is not designed to contain the refrigerated liquid in the event of
leakage from the inner container. A single containment tank system is
normally surrounded by a bund wall to contain possible product leakage.
3.2.3
Double Containment Tank System
This consists of a liquid and vapor tight primary container, which itself is a
single Containment Tank System, built inside a liquid tight secondary
container. The secondary container is designed to hold all the liquid contents
of the primary container in the event of leaks from the primary container, but it
Page 6 of 55
is not intended to contain any vapor resulting from product leakage from the
primary container. The annular space between the primary container and the
secondary container shall not be more than 6 m.
3.2.4
Full Containment Tank System
This consists of a liquid tight primary container and liquid and vapor tight
secondary container. Both are capable of independently containing the
product stored. The primary container contains the product under normal
operating conditions. The secondary container is intended to be capable of
both containing the product and of controlled venting of the vapor resulting
from product leakage from the primary container.
3.2.5
Primary Liquid Container
Parts of a tank system that contain the liquid during normal operation.
3.2.6
Secondary Liquid Container
Parts of a tank system that contain the liquid in the event of leakage from the
primary liquid container.
3.2.7
Warm Vapor Container
Parts of a tank system that contain product vapor, but are not expected to
function after exposure to refrigerated product temperature. (This includes
roofs over suspended insulation deck and the outer container of a double wall,
open top single containment tank system.)
3.2.8
Purge Gas Container
Parts of a tank system that contain only purge gas and are not expected to
function after exposure to product temperature.
(This includes outer container of double roof single containment tank system)
3.2.9
Dike Wall
A structure used to establish an impounding area which is used for the
purpose of containing any accidental spill of stored flammable refrigerants.
3.2.10 Design Pressure
The maximum gauge pressure permissible at the top of completed tank
system in its operating position for a design temperature
3.2.11 Operating Liquid Capacity
The usable volume of product, the volume available between minimum and
maximum normal operating levels that could be stored in the tank system
Page 7 of 55
3.2.12 Freeboard
The designed height above the maximum normal operating level to minimize
or prevent overflow and damage to the roof due to sloshing of the liquid
contents during a seismic (OLE, CLE or ALE) event.
3.2.13 Sloshing
A very long period of fluid motion that generates wave action on the liquid
level during a seismic (OLE, CLE or ALE) event
3.2.14 Annular Space
The space between the inner and outer shell or wall.
3.2.15 Vapor Barrier
A barrier to prevent entry of water vapor and other atmospheric gases into
insulation or into an outer tank
3.2.16 Suspended Deck
Structure suspended from the fixed roof for supporting the internal insulation
above the primary liquid container.
3.2.17 Refrigerated Temperature Roof
A roof that contains product vapor and is near the liquid product temperature
during normal operation
(This includes inner roofs of double roof tanks and single roofs of tanks with
external roof insulation)
3.2.18 Base Slab
Continuous concrete base supporting the tank system. This base may be
either on the ground or elevated. This may be either supported by soil or piles.
3.2.19 Elevated Foundation
Foundation with base slab, supported by either piles or piers (stub columns),
located at an elevation above grade leaving an air gap between the grade and
the bottom of the base slab
3.2.20 Thermal Corner Protection
Thermally insulating and having a liquid tight structure in the bottom annular
section of a tank system to protect the outer container against low
temperatures in the event of leakage from the primary container.
Page 8 of 55
3.2.21 Load Bearing Insulation
Insulation with special compressive strength properties used for base thermal
insulation and for transferring the load to the load bearing structure.
3.2.22 Base Heating System
A heating system provided in the base slab to prevent frost (0 deg C isotherm)
from penetrating the soil to cause a frost heave problem
3.2.23 Roll-over
Uncontrolled mass movement of stored liquid, correcting an unstable state of
stratified liquids of different densities and resulting in a significant evolution of
product vapor
3.2.24 Operating Basis Earthquake (OBE) or Operating Level Earthquake (OLE)
The operating basis earthquake or operating level earthquake ground motion
shall be defined as the motion due to an event with a 10% probability of
exceedance within a 50 year period (a 475 year recurrence interval). (Refer
Appendix L of API 620).
3.2.25 Safe Shutdown Earthquake (SSE) or Contingency Level Earthquake (CLE)
The safe shutdown earthquake or contingency level earthquake ground
motion shall be defined as the motion due to an event with a 2% probability of
exceedance within a 50 year period (a 2475 year recurrence interval) which is
the maximum considered earthquake in API 650 Appendix E and ASCE 7.
(Refer Appendix L of API 620)
3.2.26 Aftershock Level Earthquake (ALE)
The aftershock level earthquake (ALE) ground motion shall be defined as the
motion due to an event with a 2% probability of exceedance within a 50 year
period (a 2475 year recurrence interval), which is the maximum considered
earthquake in API 650 Appendix E and ASCE 7, with the spectral values
reduced by 50%. (Refer Appendix L of API 620)
3.2.27 Pump Column
A pipe column to house a combined vertical pump and close coupled electric
motor. The column itself protrudes through the outer tank roof.
3.2.28 Set Pressure
The pressure at which the pressure relief device first opens
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3.2.29 Set Vacuum
The vacuum at which the vacuum relief device opens to let the inflow
3.2.30 Purging
The replacement of one gas/vapor by another in an enclosed tank system by
displacement, by dilution, by diffusion or by combinations of these actions
3.2.31 Maximum Design Liquid Level
Maximum liquid level that will be maintained during operation of the tank used
for the static shell thickness determination
3.2.32 Maximum Normal Operating Level
Maximum liquid level that will be maintained during normal operation of the
tank. Normally the level at which the high level alarm is set.
3.2.33 Minimum Normal Operating Level
Minimum liquid level that will be maintained during normal operation of the
tank. Normally the level at which the low level alarm is set.
3.2.34 Boil-off
The process of vaporization of refrigerated product by heat conducted through
the insulation surrounding the tank.
3.2.35 Design Metal Temperature
The minimum temperature for which the metal components of the primary
container are designed
3.2.36 Hazard
An event having the potential to cause harm, including ill health and injury,
damage to property, product or the environment, production losses or
increased liabilities
Page 10 of 55
Draft for API Refrigerated Tank Task Group
SECTION 4 - Responsibilities
Draft No.
F
Date:
December 7, 2007
Drafted by:
Jiping Qiu
Exxon Mobil Development Company
12450 Greenspoint Dr.
Houston, TX 77060
Telephone: 281 654-3776
Fax: 262 313-4142
Email: jiping.qiu@exxonmobil.com
Writers Comments:
Changes in blue incorporate comments from Group review on December 6, 2007.
4.1
General
The owner/purchaser shall provide the specification defining the tank design per
Design Information specified below. The contractor shall be responsible for the
design, supply, fabrication, construction, inspection and testing of the tank. The
interface issues, such as pre-commissioning and other transition items shall be
resolved by the agreement between owner/purchaser and contractor.
4.2
Design Information
4.2.1
Information by Owner / Purchaser
The owner/purchaser shall provide following information:
4.2.1.1
4.2.1.2
4.2.1.3
4.2.1.4
4.2.1.5
4.2.1.6
4.2.1.7
4.2.1.8
4.2.1.9
4.2.1.10
Scope of work for contractor (including items listed in 4.2.3)
Tank type (see section 5)
Capacity (minimum net working)
Tank location on plot plan
Design life (including number of full load cycles)
Environmental data (including, minimum/maximum ambient
temperatures)
Site geotechnical and seismic data (including soil properties,
allowable soil bearing, predicted settlements after soil remediation,
and foundation type selected)
Process Flow Diagrams (PFDs), Piping & Instrumentation
Diagrams (P&IDs)
Properties of the storage product, including density at the design
temperature, toxicity, flammability
Design pressure/vacuum, maximum/minimum operating pressure
Page 11 of 55
4.2.1.11
4.2.1.12
4.2.1.13
4.2.1.14
4.2.1.15
4.2.1.16
4.2.1.17
4.2.1.18
4.2.1.19
4.2.1.20
4.2.1.21
4.2.1.22
4.2.1.23
4.2.2
Pressure relief and vacuum set points
High/low pressure alarm set point
Design Boil-off rate
Minimum design temperature of primary containment
Natural environmental loads (such as earthquake, wind)
Accidental loads (such as fire, blast/ impact)
Type of cathodic protection system (if any)
Product filling/emptying rates
Spillage handling requirements
Required rollover prevention provisions (as per 6.3.6 and 10.2.4)
Piping and instrumentation requirements (as per 7.2 and 10.2)
Corrosion allowances
Hazard protection system requirements (such as water spray, gas
detection)
Information by Tank Contractor
The contractor shall provide following information:
4.2.2.1
4.2.2.2
4.2.2.3
4.2.2.4
4.2.2.5
4.2.3
Tank total gross liquid capacity
Internal diameter and height of inner tank (ambient temperature)
Design maximum liquid level
Normal maximum/minimum operating liquid level
High/low level alarm
Agreement by Tank Owner / Purchaser and Contractor
The following issues shall be agreed by both sides.
4.2.3.1
4.2.3.2
4.2.3.3
4.2.3.4
4.2.3.5
4.2.3.6
4.2.3.7
Applicable Codes, Standards
Contractor's involvement in risk assessment
Materials of tank construction
Pre-commissioning and Commissioning procedures, including
purging, drying and cooldown
NDE applied to non-hydrotest portion of welds
Settlement prediction and inspection method
Emergency relief valve discharge flow rate
Page 12 of 55
Draft for API Refrigerated Tank Task Group
SECTION 5—Selection of Storage Concept
Draft No.
B
Date:
January 28, 2008
Drafted by:
Keith Mash
Shell Global Solutions
17 Linden Avenue
Darlington, DL3 8PS United Kingdom
Telephone: +31 70 377 2477
Mobile: +31 652 097 875
Email: keith.mash@shell.com
Chairman Comments:
Changes from draft balloted in November 2007 are in blue text.
5.1
Single Containment
The single containment concept comprises the following:

A single tank designed and constructed so that in normal operation primary liquid
and vapor containment is provided by the tank.

A tank system comprising both inner and outer containers. The inner tank
(primary liquid barrier) is designed and constructed to retain the liquid product in
normal operation. The outer container (primary vapor barrier) is provided to retain
the product vapor during normal operation and where necessary provide
protection to the insulation system.
Both concepts are designed and constructed so that only the primary vapor barrier
contains the liquid and therefore meets the low temperature ductility requirements for
cryogenic storage of liquid product.
Leakage or failure of the primary vapor barrier will lead to spreading of liquid over the
surrounding area resulting in extensive vapor formation. To limit the spreading of the
liquid and vapor cloud formation secondary containment (secondary liquid barrier) in
the form of remote bund walls are provided. The location of the bund walls from the
primary liquid barrier may vary which influences the vapor cloud formation and layout
of the facility. Either low bunds remote from the tank or higher bunds close to the wall
may be used provided that the theoretical spill volume is retained.
The primary tank is normally a freestanding vertical cylindrical low-temperature-steel
that may be enveloped by a carbon steel gas-tight primary vapor barrier. The carbon
steel metallic outer shell also provides a barrier against moisture ingress that would
otherwise lead to the deterioration and failure of the insulation system (Section 10).
Page 13 of 55
Penetrations below liquid level through the sides and bottom of the primary liquid and
primary vapor barriers are permitted. Penetrations through the liquid level in
secondary liquid barriers are not recommended; to ensure integrity of the spill
containment pipe work should be detailed to go “over” bunds as opposed to through
them. Where pipe work penetrates the bunds below the liquid levels particular
attention should be paid to the location, detailing and design to ensure that the
product is retained.
Variants of single containment concepts comprising a single tank are as follows:
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Variants of single containment concepts using independent primary liquid and vapor barriers
are as follows.
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5.2
Double Containment
The “Double containment” or “double integrity” concept evolved from single
containment and was driven by the requirement to limit the risks and consequences
of a traveling vapor cloud and potential pool fires following a liquid spill.
In order to reduce both the wetted area and surface area of a spilled pool of liquid
product the single containment system of low earth bund walls at fairly large
distances from the primary liquid barrier is replaced with close proximity high bund
walls.
Therefore the double containment tank is essentially a single containment concept
with close proximity bund walls to reduce the exposed area of spilled liquid product.
The only exception is that piping penetrations are not permitted through the primary
liquid, primary vapor and secondary liquid barriers below liquid level.
During normal operations, the inner tank provides primary liquid containment and the
outer shell provides primary vapor containment. The system is designed and
constructed so that only the inner tank contains the liquid and therefore meets the low
temperature ductility requirements for cryogenic storage of liquid product. In normal
operations the carbon steel metallic outer shell also provides a barrier against
moisture ingress that would otherwise lead to the deterioration and failure of the
insulation system.
In the unlikely event of leakage of liquid product secondary liquid containment is
achieved by the provision bund walls constructed from suitably qualified steel or prestressed concrete comprising internal or external pre-stressing systems. Alternatively
reinforced concrete may be used with an earthen embankment. In this instance the
earthen embankment provides external force and pre-stressing to the secondary
container.
The closer proximity of bund walls results in smaller area of “pooled” liquid product
compared to the single containment concept.
In comparison with the single containment, double containment differs by introducing
an additional safety barrier to contain the liquid in a smaller area and affords other
benefits in terms of blast and impact resistance. To minimize evaporation the outer
wall should generally be located at a distance not exceeding 6 m from the inner wall.
Variants of double containment concepts using single tanks and independent primary
liquid and vapor barriers are as follows:
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5.3
Full Containment
The full containment concept evolved from double containment and was driven by the
necessity to control the release of product vapors following a (postulated) inner tank
failure.
The key driver for the changes was the potential consequences associated with the
following:

Liquid product escaping from the primary liquid container

The vulnerability of the single and double containment tanks to external
threats such as blast, fire and flying hazards

Economic drivers due to the increased land requirements of single and double
containment schemes versus full containment.
Full containment tanks shall be designed so that both inner (primary liquid barrier)
and outer tank (secondary liquid barrier) are capable of containing the refrigerated
liquid. In normal operation the inner tank shall be liquid tight and retain the complete
liquid contents. The outer tank shall contain and vent the vapor resulting from
evaporation in case of a spill in a controlled manner (through the Relief or vent
system). The outer tank shall be erected at a maximum distance from the inner wall of
6 feet where provided insulation is retained in this annular space.
Full containment tanks shall be designed so that in normal operation the inner tank
(primary liquid barrier) is liquid tight and capable of retaining the complete liquid
contents. In normal operation the outer tank (primary vapor barrier) shall contain the
vapor without leakage.
The outer tank may be of metallic construction however more often than not improved
resistance to external threats and hazards is necessary and achieved through the
adoption of pre-stressed reinforced concrete.
The roof of the pre-stressed concrete outer tank may be constructed from steel or
concrete. However the steel roof is more susceptible to external hazards and threats
such as spillages, explosions, missiles and radiation and it is more conventional to
adopt a concrete roof for these tanks. Additionally where a concrete roof is provided,
the effects of a roof collapse are not credible; the resulting burn out effects need not
be considered unless stipulated or required by jurisdictional or regulatory
requirements.
Where prestressed concrete outer tanks are selected vapor containment during
normal service is achieved through the incorporation of a warm temperature metallic
or polymeric vapor barrier applied to the inside face of the concrete wall. Where
metallic outer tanks are provided vapor tightness shall be assumed.
Under emergency conditions (spillage) the outer tank (secondary liquid barrier) shall
be capable of retaining the liquid contents and vapor. Controlled release of vapor
through emergency relief systems is permitted. The outer tank shall be erected at a
Page 18 of 55
maximum distance from the inner wall of 6 ft. Where provided insulation is retained in
this annular space.
The primary liquid container has historically been constructed of welded metal plates
with suitable low temperature characteristics. Concrete primary tanks with metallic
liners have been constructed in Barcelona and Philadelphia, and concepts using only
pre-stressed concrete have been proposed. Essential to the concept of a full
containment is the assured leak-tightness of the primary liquid container. Liquid is
not permitted to accumulate outside the primary liquid container during normal
operation. Tank systems where this is not assured amount to a somewhat different
storage concept and would require consideration of such things as liquid collection
and disposal, potential cold spots, affect on tank venting, etc. This standard has not
attempted to address these issues
Normally the primary container is a freestanding vertical cylindrical tank constructed
from low-temperature-steel selected to meet the low temperature ductility
requirements at the design temperature. An outer tank envelops the inner tank
providing vapor containment in normal operation and liquid containment in the event
of an inner tank leakage.
Under inner tank leakage (emergency conditions), the vapor barrier material will be
exposed to cryogenic conditions. Vapor barrier liners are not expected to remain leak
tight in this condition; however the concrete shall be designed to remain liquid tight.
Product losses due to the permeability of the concrete are acceptable in this case.
For certain low temperature products such as methane significant design issues arise
at monolithically connected base to wall joints due to the mechanical restraint offered
by the base. To counter these it is normal practice to include a secondary bottom
protect and thermally isolate this monolithic area from the cold liquid and provide
liquid tightness.
Variants of full containment concepts using double integrity tanks are as follows:
Page 19 of 55
Page 20 of 55
5.4
Guidance on Selection of Storage Concept
5.4.1
Introduction
By definition petrochemical facilities involve processes which could in the
event of an upset or emergency event release hydrocarbons that present a
significant threat to human life, the environment and surrounding communities.
This is especially true for cryogenic gases where gaseous liquids can be up to
600 times their liquid form. The potential result following a spillage is
generation of large vapor clouds that can drift in an uncontrolled manner
across the facility.
The proposed site for the facility and should specifically address the impact of
gas clouds and heat radiation on local and adjacent facilities. Intrinsic within
this approach is the selection of storage concept, separation distances, the
proximity to property lines, site topography; soil conditions, and ground water
conditions. A review of the site may identify constraints or provide
opportunities to utilize specific features of site to the benefit of the facility i.e.
natural topography may drive the selection of single containment selection.
The possible effect of a liquid spill from any portion of the facility should be
considered. This is particularly relevant where there is a conduit through which
the liquid product can flow beyond the secondary liquid barriers.
The determination of vapor generation and dissipation is complex and
dependent on many parameters including relative gas to air densities,
meteorological conditions, terrain, rate of release and the amount of entrained
liquid droplets dispersed into the vapor cloud.
When liquid product spills from evaporation takes place. The amount of
evaporation in the very first moments depends primarily on the exposed
surface in contact with the cryogenic liquid.
Initially the refrigerated hydrocarbon vapors are heavier than air and sink due
to their low temperature. However as heat is drawn from the environment
some hydrocarbons become less dense and when warmed eventually
become lighter than air. Propane and heavier vapors remain denser than air,
even when warmed to ambient temperatures. In this respect full containment
tanks are particularly well suited for heavy volatile gases such as Propane and
Ethylene.
Depending on atmospheric and environmental conditions the resulting gas
cloud may drift across or away from the facility prior to being dispersed below
its lower flammable limit where ignition is no longer possible. The area wetted
in the case of a spill should therefore be limited in order to reduce the size and
travel distance of a gas cloud. The exposed wetted area is directly linked to
the selection of containment concept.
The heat generation from a large pool of burning liquefied gas is significantly
higher than that of a similar pool of another oil product. Again in order to limit
Page 21 of 55
the heat radiation on the surroundings to acceptable levels it may be
necessary to reduce as much as possible the area of the pool of spilled
liquefied gas though the selection of containment concept.
Consideration should be given to the proximity of storage tanks to highly
populated areas; the risk of external events and threats (such as earthquakes
and falling aircraft) that may effect the integrity if the tanks; jurisdictional and
regulatory requirements and constraints of land availability.
The most appropriate method to achieve this is by means of a staged risk
assessment that clearly identifies the hazards, failure conditions, probabilities
of occurrence and consequences thereof.
5.4.2
Hazard Identification
The storage concept should be selected by PURCHASER following a risk
assessment. The hazards assessment shall consider the requirements of
national and Local Authorities and the influence of external, internal and
environmental hazards. Key drivers known to influence storage concept
selection are as follows.









Product to be stored
Availability of land
Earthquakes
Blast loading
Fire loading
Projectiles
Proximity to residential developments and habitable areas
Influence/impact of adjacent process plant and equipment
Relief valve fires
Additional hazards that should be considered by the PURCHASER when
selecting a containment concept are as follows. This list is not exhaustive and
a risk assessment should be performed out by responsible parties to identify
the critical hazards that influence the concept selection and plant layout.
All containment systems shall be designed with the assumption that the
primary tank fails and gradual filling of the secondary container takes place.
For metallic tanks where the material is selected and tested in accordance
with this standard the possibility of sudden failure “unzipping or zip failure” of
the inner tank is not a normal design consideration, but in cases where the
purchaser specifies that it should be taken into consideration, it is essential
that the outer tank or wall is designed to withstand the consequent impact
loading.
The risk assessment shall demonstrate that the risks to property and life are
acceptable, both inside and outside the plant boundary.
The risk assessment process commences with identification of the hazards
that can be grouped into external and internal threats.
Page 22 of 55
5.4.2.1
External Hazards









5.4.2.2
Internal Hazards










5.4.2.3
Environmental hazards including earthquake, lightning, wind
loading including hurricane/typhoons, flooding, snow and ice
loading, tsunamis and seiches.
Ground conditions, weak strata, liquefiable layers, lateral
spreading, presence of caverns, voids and defects.
Flying objects, including missiles and equipment following a
process incident
Falling objects including helicopters, planes and proximity of
airports/flight paths
Pressure waves due to explosions from the process plant,
adjacent plant, process equipment and carriers. Including
facilities located outside the boundary limits.
Operational and upset conditions including spillage and
leakage of product
Maintenance hazards;
Fire hazards from adjacent tanks, bunds, relief valves, sumps,
jet fires and plant areas
Proximity of tanks to external uncontrolled sources of ignition
such as ground flares, flares.
Leakage of product from the inner tank
Overfilling of the tank i.e. at export facilities with high run down
rates to single/multiple tanks
Over/Under pressurization of the tank due to process upset
Roll-over leading to over pressurization of the tank
Major leak i.e. the complete failure of the inner tank
Minor leak partial leakage from the inner tank due to a
postulated defect
Fatigue and cyclic loading of key components e.g. annular
plates
Corrosion
Failure of pipe work attached to the tank bottom/sides
Instrumentation failures
Safety Improvement
Where the hazard assessment identifies risks that are out with
acceptable limits then positive measures (action) shall be taken to
reduce the level of risk to an acceptable level. Such mitigation
measures may be as follows;


Selection of alternative containment concepts i.e. migration
from single containment double or full containment.
Improvements to process equipment selection
Page 23 of 55





Substitution of a steel roof on a full containment tank with a
concrete
Increase in safety distances (separation distances) to limit
impact in respect of vapor dispersion and impinging radiation
Elimination of ignition sources
Selection of alternate layouts and site locations
Inclusion of protection systems to shield/protect critical
equipment from hazards
Page 24 of 55
DRAFT FOR API REFRIGERATED TANK TASK GROUP
SECTION 6 - Design Considerations
DRAFT NO. C
Date: January 14, 2008
Drafted by: Sheng-Chi Wu with Jack Blanchard, Rama Challa and Anant Thirunarayanan
Revisions in blue from December 6, 2007 meeting – J Blanchard and Rama Challa
6.
General (drafted by Sheng-Chi Wu)
This section provides guidance for design of refrigerated tank systems when
subjected to applicable normal and abnormal design loads defined in 6.4, and to meet
the performance criteria prescribed in 6.3. Guidelines for performing the seismic
analysis are presented in 6.5.
6.1
Spacing Requirements (drafted by Jack Blanchard / Anant Thirunarayanan)
Spacing of refrigerated gas storage tank systems from adjacent property and
adjacent tanks should consider the following:


Thermal radiation from a liquefied flammable gas pool fire contained by the tank
system secondary liquid containment. The secondary liquid containment of an
adjacent tank system shall maintain its ability to contain liquid.
Dispersion of gas evolved from liquefied flammable gas contained by the tank
system secondary liquid container to a non-flammable mixture.
Acceptable guidance for satisfying the above for LNG storage can be found in NFPA
59A.
6.2
Maximum Design Liquid Level (drafted by Anant Thirunarayanan)
The tank shell shall be designed to the design liquid level. This level is higher than
the normal maximum operating level of the tank. The shell height shall account for the
sloshing wave height for tanks subjected to seismic conditions. The net capacity of
the tank for refrigerated product is the volume contained between the normal
maximum and minimum operating levels. These issues are illustrated in Figure 6-1.
Page 25 of 55
O perating F reeboard
D es ig n L iquid L ev el
S eis mic F reeboard
S ee 6.5.9
Normal Maximum
O perating L ev el
G ros s
C apac ity
Net C apac ity
Normal Minimum
O perating L ev el
Figure 6-1 - Liquid Levels
6.3
Performance Criteria (drafted by Jack Blanchard)
Design and erection of tank systems in accordance with this standard shall satisfy the
performance criteria of this section.
6.3.1
Normal Operating
The primary liquid container shall contain the liquid under all normal operating
loads and conditions. Refer to section 5 for further definition of the meaning of
primary liquid containment for various storage concepts.
The primary vapor container shall contain the vapor and be 100% vapor tight
during normal operation. Further it shall have adequate pressure capacity
above normal operating pressure to prevent venting during normal operating
conditions.
6.3.2
Abnormal and Emergency Conditions
The primary liquid container shall be designed to maintain liquid containment
even under the abnormal and emergency conditions specified herein.
However if for any unforeseen condition the primary containment is not
maintained, then secondary containment shall contain all liquid product.
Vapor containment requirements for abnormal and emergency conditions vary
depending upon the storage concept specified; see Section 5.
Refer to ACI 376 Sections 4.1 and 4.2 for detailed criteria related to leak
tightness of concrete primary and secondary containers.
The secondary liquid container shall be sized to contain the contents of the
primary liquid container when at the normal maximum liquid level.
Page 26 of 55
6.3.3
Commissioning and Decommissioning
The tank system shall allow the criteria in Section 10 to be met. In addition,
the tank system shall be capable of being decommissioned including purging
to a gas to air mixture considered safe for personnel access.
6.3.4
Boil-off
The tank insulation system shall limit the boil-off rate to below the rate
required by the plant design or maximum specified by the owner. The boil-off
rate, normally specified in percent per day of maximum normal operating
volume assuming a pure product, (unless otherwise specified by owner) shall
be based on climatic conditions as specified for the project.
Climatic conditions normally considered in the design include:



6.3.5
Highest recorded average temperature (over 24 hour period)
No wind
Solar radiation effects
Roll-Over
For stored products subject to rollover condition, the tank system shall provide
a means to prevent roll-over. See section 7.4.4 for active management of the
stored product when rollover conditions are determined to be applicable.
6.3.6
Design Temperature

Primary and secondary liquid containment tanks and process lines
carrying liquid or gas.
- Minimum design temperatures shall be selected no higher than the
pure product boiling temperature at one atmosphere (See Appendix
A). However, design conditions such as introduction of sub-cooled
product may require a lower design metal temperature.

Primary vapor containers subjected primarily to product temperatures.
- Minimum design temperatures shall be equal to the design
temperature for the primary liquid container.

Primary vapor containers subjected primarily to ambient temperatures.
- Minimum design temperatures shall be equal to the lowest one day
mean ambient temperature for the geographic location.

Purge gas containers, not subject to containment of product vapor:
- Minimum design temperatures shall be equal to the lowest one day
mean ambient temperature for the geographic location.
Page 27 of 55

6.3.7
Local areas of the primary vapor container exposed to lower than ambient
conditions.
- Areas such as process nozzle thermal distance piece connections to
the vapor container may be subjected to temperatures below ambient
conditions. The design temperature for these locations shall take this
local cooling effect into consideration.
Differential Movements
The design of the tank system shall provide for differential movements
between tank components resulting from differential design temperatures and
erection vs. operating temperature. Components that are restrained from free
differential movement shall be designed to incorporate adequate flexibility to
maintain structural integrity.
6.3.8
Foundation Settlement
Design conditions for Liquefied Gas storage tank system foundation, including
the supporting soil, shall include for predicted settlements. The following
components shall be designed for predicted settlements (both short term and
long term):






the bottom insulation system
the metal or concrete primary liquid container
the concrete outer wall of full containment refrigerated gas storage tanks
post-tensioning system
the various tank attachments
piles or other structural support systems
Annex B provides guidance for evaluating settlements.
6.3.9
Protection from Freezing of Soil
Where freezing of liquid in the soil under the tank foundation is possible and
could cause heaving of the soil, the tank foundation design shall include a
means to maintain the soil at a temperature above 32oF (0oC), or to separate
the cooling effect of the tank from the supporting soil.
6.3.10 Seismic Performance
Tanks designed and built to this standard shall be designed for three levels of
seismic motion. The magnitudes of the seismic ground motions are defined in
Section 6.4.2 and sliding resistance requirements are defined in Section
6.5.10. In addition, the design shall meet the requirements of the applicable
local building codes.
a. Operating Basis Earthquake (OBE): The tank system shall be
designed to continue to operate during and after OBE event. The OBE
is also referred to as Operating Level Earthquake (OLE) in API 620,
App L.
Page 28 of 55
b. Safe Shutdown Earthquake (SSE): The tank system shall be designed
to provide for no loss of containment capability of the primary container
and it shall be possible to isolate and maintain the tank system during
and after SSE event. The SSE is also referred to as Contingency
Level Earthquake (CLE) in API 620, App. L.
c. Aftershock Level Earthquake (ALE): The tank system, while subjected
to ALE, shall provide for no loss of containment from the secondary
container while containing the primary container volume at the
maximum operating liquid volume.
The primary liquid container shall contain the seismic sloshing wave as
defined in Section 6.5.9.
6.4
Design Loads and Load Combinations (drafted by Rama)
6.4.1
Design Loads
The following types of design loads shall be considered in the design of the
liquefied gas containers and foundations. API 620 and ACI 376, Chapter 3
provide guidance on the types of the design loads and load combinations to
be used. They include, but are not limited to the following:
Normal Loads:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Dead loads
Product Pressure and Weight
Internal pressure
External pressure
Construction-Specific Loads
Testing and Commissioning Loads such as test, vacuum and pneumatic
tests
Thermally-induced loads experienced during tank purging, cooling and
filling
Shrinkage and Creep-Induced Loads
Pre-stressed loads for concrete container
Live Loads
Environmental Loads such as Snow, Wind and ice loading
Seismic Loads (OBE, defined in Section 6.4.2)
Decommissioning Loads
Loads induced by predicted differential settlement
In addition to the normal loads indicated above, the following loads from
abnormal events shall be considered in the design.
1.
2.
3.
Loads from liquid spill condition
Loads based on a risk assessment such as fire, blast, external missile
etc.
Seismic Loads (SSE & ALE, defined in Section 6.4.2)
Page 29 of 55
6.4.2
Seismic Loads
Probabilistic seismic hazard studies are required to determine the seismic
ground motions for design of tank-fluid-foundation system. Three levels of the
seismic ground motions shall be considered:
a. Operating Basis Earthquake (OBE): The OBE is defined as the seismic
ground motion having 10% probability of exceedance within 50 year
period, i.e. 475 year recurrence interval.
b. Safe Shutdown Earthquake (SSE): The SSE is defined as the seismic
ground motion having 2% probability of exceedance within 50 year
period, i.e. 2475 year recurrence interval.
c. Aftershock Level Earthquake (ALE): The ALE is defined as half of the
SSE.
6.4.3
Load Combinations
The design loads shall be combined to produce load combinations to be used
in the analysis and design of the liquefied gas containers. Load combinations
are dependent on the material type of the container. API-620 provides
guidance on the load combinations to be used for Liquefied gas metal
containers and ACI-376 for Liquefied gas concrete containers.
6.5
Seismic Analysis (drafted by Jack and Sheng)
6.5.1
General
The tank system shall be designed for three levels of seismic ground motions
as defined in Section 6.3.14 and 6.4.2 above. The rules in API 620 Appendix
L shall be applied to all steel tanks designed to this standard. The rules of
ACI 376 shall be applied to all concrete tanks designed to this standard.
6.5.2
Site-Specific Response Spectra
The site-specific horizontal and vertical acceleration response spectra shall be
developed for both OBE & SSE for different damping values of up to 20%.
6.5.3
ALE Design
The ALE earthquake shall be considered only for the seismic design of
secondary containment with full liquid condition, assuming that the primary
container is damaged after the SSE event.
6.5.4
Tanks Supported on Rock
When the tank foundation is supported on rock-like site (defined as the site
class A & B in IBC or ASCE 7), the fixed base condition is considered. In this
case, the structural damping values shall be used for determining the seismic
responses (SSI is not considered)
Page 30 of 55
6.5.5
Soil-Structure Interaction
When the tank foundation is supported on soil site (defined as the site class C
to F in IBC or ASCE 7), soil-structure interaction seismic analysis (SSI) should
be applied. In this case, dynamic soil and pile stiffness and damping
parameters shall be included in the tank model for SSI analysis. Dynamic
soil/pile properties are evaluated by considering the effects of seismically
induced soil strains and forcing frequencies. System damping for SSI shall be
calculated for determining seismic response, and should be limited to15% for
OBE, and 20% for SSE.
6.5.6
Response Modification Factors – OBE
In order for the tank system to remain in continuous operation during and after
OBE, the elastic method of seismic analysis should be performed. The
response modification factor, R, applied in the response spectra design
method shall be 1.0.
6.5.7
Response Modification Factors – SSE
The response modification factors for SSE design applying the response
spectra design method shall meet those specified in API 620 L Tables L-4Q
and L-4R for steel tanks and ACI 376 Section 6 for concrete tanks. Response
reduction factors are not applicable for non-linear dynamic analysis methods
incorporating fluid-structure and soil-structure interaction.
6.5.8
Seismic Design Liquid Height
If the tank system includes high level alarms to restrict the maximum normal
operating level to a level lower than the maximum design liquid level, the
maximum normal operating level may be applied to all seismic design
including freeboard determination.
6.5.9
Seismic Sloshing Height
The seismic sloshing wave height shall be calculated in accordance with API
620 Appendix L. The freeboard height shall be determined based on the OBE
sloshing height plus 1 ft allowance or the SSE sloshing height, whichever is
larger.
6.5.10 Resistance to Base Shear – Sliding
The rules in API 620 Appendix L shall be applied to determine sliding
resistance. In high seismic regions a more extensive analysis may be applied,
provided it includes evaluation of the response of the shell, the fluid, and
foundation (in the case of a slab) to the fluctuation of liquid pressures in the
tank. When applying this approach, the horizontal and vertical seismic
response should be applied based on the component combination of 100%
and 40%. The case for the 100% vertical plus 40% horizontal load case shall
be evaluated in addition to the 100% horizontal plus 40% vertical load case
Page 31 of 55
defined by API 620, App L. Alternately, a time history analysis approach may
be applied incorporating both horizontal and vertical motions simultaneously.
6.5.11 Evaluation of Damage from an Earthquake
The seismic design may assume that when a tank system is subjected to an
earthquake exceeding an OBE magnitude event, the tank system will be
evaluated for permanent distortion, continued safe operation, and the need for
repairs.
6.5.12 Interaction Between Tank and Adjacent Structures
Consideration for flexibility of components connecting the tank system to
adjacent structures shall be included in the tank system design.
6.6
Foundation Design (drafted by Rama Challa)
6.6.1
General
Tank systems for liquefied gases shall be installed on suitable foundations
designed to transmit all loadings to suitable load bearing soil strata. Types of
foundation support systems consist of Raft or Mat foundations, pile
foundations (steel H-piles, cast in-situ concrete piles or precast prestressed
concrete piles) and elevated foundations supported on drilled shafts or vertical
walls. ACI 376 Chapter 8 provides guidelines on foundation design.
Foundation support systems are dictated by detailed geotechnical
investigation of the location for siting of the Tank Systems. The extent and
detail of the soil investigation shall be specified by qualified geotechnical
engineers. ACI 376 Section 8.2 gives detailed guidelines on the
geotechnical investigations to be performed.
The materials of construction and the foundation type shall be designed to
adequately resist the operating and emergency temperature conditions. The
foundation shall maintain integrity under normal operating conditions. One
method of maintaining the foundation integrity is to utilize the foundation base
heating for grade supported mat foundations where subsoil freezing would
occur under normal operating conditions. Elevated foundations with adequate
air gap between the bottom of the foundation and grade shall be considered in
cases where base heating methods are not feasible.
6.6.2
Ground Improvement
Ground improvements shall be considered in unsuitable areas encountered in
the tank siting. ACI 376 Section 8.5 gives detailed guidelines on the ground
improvement techniques.
6.6.3
Design Codes
Page 32 of 55
ACI 318 is utilized for the structural design of concrete elements utilized in the
foundations. The concrete elements include Raft or Mat foundations, cast insitu concrete piles or precast prestressed concrete piles, drilled shafts and
vertical walls. For concrete elements in direct contact with liquid product, ACI
350 may be utilized. AISC steel construction manual provides guidelines on
design of steel H-Pile Sections.
6.6.4
Factors of Safety
In general, the design soil parameters, viz., bearing capacity, pile compression
and tension capacities etc., are based on allowable stress design (ASD)
approach with an ultimate capacity value divided by a factor of safety. The
factor of safety is based soil type, variability and soil characterization. The
factors of safety presented in Section 8 of ACI 376 can be used as a guide.
The actual factor of safety shall be determined on a case by case basis,
based on the recommendations of geotechnical engineer in charge of
geotechnical investigations. An alternate design approach could be applied by
comparing the ultimate factored soil/pile loads with the ultimate soil bearing or
pile capacities.
ACI 318 utilizes an ultimate design strength approach for the design of the
concrete foundation slabs, piers or piles.
6.6.5
Mat Foundations
If the supporting soil is found adequate with respect to load transfer and
settlements then a soil supported mat foundation may be used. Adequate
consideration shall be given to the effects of differential settlement, shrinkage,
creep and thermal strains.
6.6.6
Pile Foundations
If bearing capacities are not sufficient or estimated settlements are excessive,
then the foundation base slab may be supported by piles. Negative skin
friction (down drag) and pile interaction (group effects) shall be considered
when determining the foundation capacity. The pile design shall consider
operating loads, tank settlements, thermal cycling, drying shrinkage, creep
effects, lateral deformations from wind, OBE and SSE earthquakes.
6.6.7
Anchorage (drafted by Jack Blanchard)
Anchorage of primary or secondary metallic containment tanks shall consider
the following:



Differential movement between the anchorage and the connection to the
container
Local stresses at the connection to the container
Differential strength along the length of the anchor due to thermal effects and
weld materials
Page 33 of 55


6.7
Connection details where the anchor extends through a containment boundary
(such as the secondary containment bottom of a full or double containment
tank)
The anchorage shall exhibit ductile behavior prior to failure. Connections of
the anchors to the container and foundation shall be designed for 1.25 times
the anchor capacity at the minimum specified yield stress increased to
account for thermal increases in material properties at design temperature.
Thermal Corner Protection System (TCP) for Concrete Tanks (drafted by Jack
Blanchard)
Where required by ACI 376, the design of the wall to slab junction of a concrete
container shall consider the effects of differential movement between the wall and
base. Design of the junction shall also consider the application of differential thermal
stresses and prestress forces to provide liquid containment in case of a spill.
A standard solution applies a metallic thermal corner protection expansion joint and a
secondary bottom. The TCP may be designed to withstand the full hydrostatic
pressure from a full spill, or may transfer a part of the pressure to the wall through
load bearing insulation. If a TCP is required, the following shall be considered in the
design of the TCP:






The location of the top of the TCP as related to the prestress force diagram.
Differential thermal movements between the connection to the wall and secondary
bottom including the following conditions: Operating, small spill, full spill, and full
spill plus ALE.
Differential movements due to wall prestress and creep
Wall rotation due to foundation settlement
Differential shrinkage between the wall and top of TCP connection
Erection tolerances between the TCP and the load bearing insulation
Page 34 of 55
Draft for API Refrigerated Tank Task Group
SECTION 7 - Accessories and Appurtenances
Draft No.
F
Date:
Dec. 27, 2007
Drafted by:
Larry Hiner and Masaki Takahashi
Larry Hiner
Chicago Bridge and Iron Company
14105 S. Route 59
Plainfield, IL 60544-8984
Telephone: 815 439-6125
Fax: 815 439-6654
Email: LHiner@CBI.com
Masaki Takahashi
IHI Inc.
Telephone: 713-270-2644
Fax: 713-981-2888
Email: masaki.takahashi@akerkvaerner.com
This revision incorporates the following changes based on balloting for the Dec 6 TG meeting
in Houston
1. Shell manholes in full containment tanks are disallowed accept when specified by the
purchaser. When shell manholes are used on the primary liquid container for any
configuration, they shall utilize welded closures.
2. Secondary roof egress systems are required if the primary access can be obstructed
unless waived by the Purchaser.
3. No flanges in piping are allowed between the inner and outer tanks.
4. Leak detection may not be required for the single containment tank unless Purchaser
specifies or required as a result of a hazard study.
Key: Text in blue are changes to the balloted version resulting from task group comments as
agreed upon during the December 6, 2007 meeting.
7.0
General
Accessories and appurtenances considerations for safe operation of the tank are addressed
in the following paragraphs.
7.1
Access
7.1.1
Tank Interior Access
Shell manholes shall not be used in the primary liquid container of full
containment tanks unless otherwise specified by the Purchaser. In other tank
configurations, shell manholes may be provided in the primary liquid container
and, when used, shall utilize welded closure details to prevent leakage during
service.
Page 35 of 55
7.1.2
Tank Roof Access
A primary system for accessing the tank roof shall be provided. The type of
roof access system shall be suited for reliable personnel ingress /egress.
Unless otherwise specified by the Purchaser, a second access system shall
be provided if the primary tank egress pathway can be obstructed or if the
primary system is mechanically operated and powered (e.g., electrical or
hydraulic elevator).
Walkways or platforms shall be provided to access all roof appurtenances
requiring periodic maintenance such as vents and level gauges and for access
to the roof manholes.
7.2
Process Piping
7.2.1
Process Piping - General Requirements
7.2.1.1
Material requienments - Refer to API 620 Sections Q.2.5 or Table R1 for material selection requirements for process piping components.
7.2.1.2
Nondestructive examination - Refer to API 620 Sections Q.5.7 or
R.5.7
7.2.1.3
Configuration requirements
a.
Flanged joints in refrigerated liquid and vapor piping systems
are not permitted in the space the between inner and outer
containers.
b.
For single containment tanks, process lines may penetrate the
roof, bottom, or shell except as restricted by specification or
regulation. In-tank valves should be considered when bottom
or shell process lines are used. The in-tank valves shall be
automatically activated on pressure drop due to failure of
external piping and shall also be automatically activated during
a loss of electrical power or shall be capable of being activated
from a remote location.
c.
For double or full containment tanks, shell or bottom
penetrations that breach the primary and secondary
containment are not allowed. This requirement may be waived
by the Purchaser unless required by regulations (such as
NFPA 59A). When waived, all of the following requirements
shall be met:
i.
A secondary containment dike (for external line break
before the isolation valve) is provided
ii. In-tank valves are provided when storing flammable
products Refer to 7.2.1.3(b)
Page 36 of 55
7.2.2
Tank Fill Lines
Fill lines may be top and/or bottom fill type (except as limited by 7.2.1.3) as
required by process conditions and roll-over mitigation as per 7.4.4.
7.2.3
Tank Outlet System
When shell or bottom outlets are not provided, pump columns and in-tank
pumps are required. Pump columns, extending from above the roof level to
near the tank bottom, shall be designed to transport product to the outlet line
connection on the roof and to contain the removable pump. The pump
columns shall be designed, constructed, and tested in accordance with ASME
B31.3 or ASME Section VIII. Pump columns shall be designed for pump
removal and replacement during tank operating conditions by transporting the
pump through the inside of the pump column.
7.2.4
Purge System
Purge system including vessel connections shall be provided to facilitate
Section 12 on purge and cool down
7.2.5
Cool-Down System
The tank system shall include a separate fill line specifically for cool down of
the tank. The system shall have a means for control of the flow to meet the
cool down rates defined in Section 10. For products stored at temperatures
below -510 C (-600 F), the cool down line shall incorporate spray nozzles and
shall introduce liquid near the top center of the primary liquid containment
tank.
7.3
Relief Valves
7.3.1
General
Design and installation of tank pressure and vacuum relief valves shall comply
with API620 Sec.9 and API 2000 (and other applicable code and standard,
such as NFPA58, NFPA59, NFPA59A, ANSI K61.1, etc.).
Conditions related to the plant process design, as determined by the plant
process designer shall be evaluated.
Venting requirements of this standard shall be met by relief to atmosphere. If
release to atmosphere is not allowed for the product stored, a second set of
relief valves shall be provided to flare, set at a lower set pressure.
7.3.2
Pressure Relief Valves
The number of pressure relief valves required shall be calculated based on
the total product vapor outflow and the applicable set point considering flow
losses from the inlet and vent piping of the relief system. In addition, one
spare valve shall be installed for maintenance purposes.
Page 37 of 55
The inlet piping shall penetrate the suspended deck where applicable but be
located above the level of the top of a primary liquid container, thus preventing
cold vapor from entering the warm space between outer roof and suspended
deck under relieving conditions.
Required capacity shall be based on the largest single relief flow or any
reasonable and probable combination of the following relief flows.
Reserve capacity relief valves may cover some of the following conditions if
owners specify.
a.
b.
c.
d.
e.
f.
g.
h.
Fire exposure
Operational upset
Failures at interconnected facilities
Heat input from pump recirculation, if any
Barometric pressure change
Tank filling
Tank heat leak (boil off)
Roll over, if required by owners or applicable regulations
In addition to above conditions, following conditions should be considered for
the full containment tank.
i.
j.
7.3.3
Vapor generated due to a primary liquid container leakage (Note:
EN14620-1 section 7.2.2.1 may be referred to for sizing of the relief
flow.)
Overfill, if it is required by hazard study.
Vacuum Relief Valves
The number of vacuum relief valves shall be calculated based on the total air
inflow and set points specified. In addition, one spare valve shall be installed
for maintenance purposes.
The Vacuum relief valves shall allow air to enter the vapor space. If the air
enters the cold vapor space directly, the flow rate due to temperature change
of the air should be taken into consideration.
Required capacity shall be based on the following:
a. Withdrawal of liquid and vapor
b. Barometric pressure change
7.4
Instrumentation
7.4.1
Level Gauges / Over Fill Protection
The tank shall be equipped with two independent liquid level gauges, which
include high level alarms. Third high-high liquid level alarm and cutoff device
shall be required separately. All level instruments shall be designed and
installed so that they can be maintained during operating condition.
Page 38 of 55
7.4.2
Leak Detection
A leak detection system for the primary liquid container shall be provided for
double wall tanks. However, it may not be required for the single containment
tank unless Purchaser specifies or required by a result of a hazard study. The
system shall be based on one of the following system.
a.
b.
c.
7.4.3
Temperature drop
Gas detection
Differential pressure measurement
Temperature
Temperature monitoring devices for primary liquid container shall be provided
to assist in controlling cool down and to monitor liquid and vapor temperature
as required for process.
7.4.4
Roll-Over
Roll-over conditions shall be prevented by active management of the stored
liquid. If roll-over is determined to be applicable per section 6.3.5, a density
measurement system shall monitor the density over the full liquid height and
give an alarm when predicted roll-over conditions are approached. Predicted
roll-over conditions shall be prevented or resolved by mixing the liquid, such
as by appropriate top and/or bottom filling or by recirculation.
7.4.5
Pressure
Two pressure instruments are required to monitor and control tank pressure.
The pressure instruments shall be connected to the space above maximum
liquid level.
7.5
Foundation Accessories
7.5.1
Foundation Heating (when required by foundation design)
The foundation heating system shall be designed to meet the performance
criteria mentioned in 6.3.9. The heating system shall be controlled by
temperature sensors, which are installed in the foundation. The foundation
heating system shall have 100% redundancy and give an alarm for the system
failures.
7.5.2
Foundation Settlement Monitoring
In monitoring the settlement, an independent datum reference point located
beyond the influence of local foundations shall be established. Permanent
markers shall be provided to facilitate settlement monitoring around the
perimeter of the foundation at a minimum of 8 locations not more than 30 ft
apart. In addition, for concrete outer wall tanks and for settlement conditions
that are expected to approach the design limits set for the tank, provisions
Page 39 of 55
shall be made to measure dishing settlement. An inclinometer system can be
provided to accomplish this requirement.
7.5.3
Seismic Ground Motion Measurement
As noted in section 6.5.11, seismic design may assume a damage evaluation
is performed for seismic events greater than an OBE event. A seismometer
around the tank(s) or in the plant may be required to determine OBE
exceedance.
7.6
Fire and Gas
7.6.1
General
Protections for fire and gas should be required as per 7.6.2 to 7.6.5, if they are
specified by the purchaser or per regulations, or as a result of a hazard study.
7.6.2
Fire and Gas Detection
For tanks designed to store flammable products, flammable gas detector(s)
should be installed in the area where the potential leakage could happen (e.g.
flange joints in the area of the process line on the tank and tail pipe of PRV).
7.6.3
Fire Protection
All essential appurtenances and equipments on the tank and the platform
should be protected against heat radiation by mean of fixed cooling water
spray systems, bund walls, fire proofing or other relevant method enabling
protection against thermal radiation resulting from external fire.
7.6.4
Heat Detection
Heat detector(s) should be provided to give an alarm so that the water spray
system mentioned in 11.4.2 can be activated at the area wherever the water
spray is used for fire protection.
7.6.5
Low Temperature Detection
Low temperature detector(s) should be installed at the area where potential
leakage could happen.
7.7
Electrical
7.7.1
Lightening Protection
Lightning protection shall be provided in accordance with NFPA 780 (and
NFPA 59A as applicable).
7.7.2
Grounding / Earth
Tank grounding shall meet the requirements of NFPA 780.
Page 40 of 55
7.7.3
Aviation Lighting
Aircraft warning lights shall be supplied when required by FAA or applicable
local/international rules and regulations or when specified by the purchaser.
7.8
Cryogenic Liquid Spill Protection
7.8.1
General
Cryogenic liquid spill protection shall be considered as per 7.8.2 and 7.8.3, if
specified by the purchaser or per regulations, or as a result of a hazard study.
7.8.2
Roof Spill Protection
A cryogenic liquid spill protection system should be provided to prevent roof
damage. This system may consist of a separate collection system or utilize
roof components, constructed of suitable cryogenic material. For example, a
concrete roof may be a part of the spill protection system if enough concrete
cover is provided or cryogenic re-bar is used. Design spill parameters shall be
per the applicable regulation and any purchaser supplemental requirements.
7.8.3
Cold Protection of Structures for Pipe Support
Structures used for support pipe, whose stability is essential to plant safety,
should be resistant to or protected against cryogenic liquid if they are subject
to exposure.
7.9
Miscellaneous
7.9.1
A means for handling roof top equipment requiring periodic maintenance, such
as in-tank pumps, shall be provided.
7.9.2
Perlite Fill Nozzles
Perlite fill nozzles in the roof shall be when applicable provided per 9.5.3.5.
Page 41 of 55
Draft for API Refrigerated Tank Task Group
SECTION 8 – Quality Assurance and Quality Control
Draft No.
B
Date:
January 2, 2008
Drafted by:
Rama Challa
8.1
Introduction
Liquefied Gas Storage Tank construction consist of various sub activities such as
design, procurement, fabrication, construction and testing for all subsystems such as
the foundation, tanks, piping, insulation, electrical and instrumentation systems etc. A
quality management system shall be utilized to ensure that the work performed meets
quality requirements.
Quality management systems consist of Quality Control (QC) and Quality Assurance
(QA) activities.
Quality Control (QC) is a system of routine activities developed to measure and
controls the quality of the work as it is being performed. The QC system is designed
to provide routine and consistent checks to ensure correctness and completeness
and identify corrective responses.
Quality Assurance (QA) consists of a series of systematic planned activities
implemented in a quality system so that quality requirements are met. QA includes
review procedures for the implementation of QC.
ISO 9001 & API Specification Q1 provide guidance for establishing QA and QC plans.
8.2
NDE, Testing and Tolerances
Non Destructive Evaluation (NDE) activities during Liquified Tank construction shall
be followed to ensure that the quality requirements of the work are met. Testing such
as hydrostatic and pneumatic tests, loop checks of the electrical work, foundation
level measurements etc. are mandated by construction standards to ensure
construction integrity. Tolerances are imposed by the construction codes such that
design considerations are not violated.
The Requirements for construction summarized in design and construction standards
such as API-620, ACI 318, ACI 376 and ASME B31.3 shall be followed during
construction. In addition to the standards, additional requirements may be imposed
by regulatory codes such as NEC and NFPA 59A.
Page 42 of 55
Draft for API Refrigerated Tank Task Group
SECTION 9 - Insulation
Date: January 1, 2008
By:
John Mooney
Pressure Vessel and Tank Consulting
2512 Veranda Lane, Greensboro, NC 27455
phone: 336-282-4003 email: mooneyjohnl@bellsouth.net
9.1
System Design
The insulation system shall be designed to:
a. Not fail under the specified or calculated static and dynamic loads
b. Maintain product boil-off at or below the specified limit at the specified climatic
conditions.
c. Maintain components (such as those of an outer tank) at or above their
minimum design temperature.
d. Minimize condensation and icing
e. Prevent soil freezing (in combination with the tank foundation heating system
for foundations on grade).
f. Prevent ingress of moisture (in combination with other tank components).
g. Be purged (loose fill and blanket insulation) during commissioning and
decommissioning.
9.2
9.3
Materials
9.2.1
Tests of materials are required to ensure that their properties (thermal
conductivity, strength, density, etc.) are adequate, except for Perlite which is
field tested. See 10.8 for specifications.
9.2.2
A detailed testing, installation and inspection plan shall be submitted by the
tank manufacturer to the purchaser.
9.2.3
Insulation shall be protected, particularly from moisture, during shipment,
storage, installation, tank hydrotest and in service.
9.2.4
For liquid oxygen tanks, insulation shall be non combustible.
Load Bearing Bottom and Thermal Corner Protection (TCP) Insulation
9.3.1
System Design
9.3.1.1
The insulation must be designed for static and dynamic
compressive and shear loads. These loads include weight,
earthquake, and tank movement due to commissioning and
decommissioning and filling and emptying.
Page 43 of 55
9.3.1.2
9.3.2
9.3.3
For secondary liquid containment concrete tanks with walls
attached to the foundation slab, the lower part of the wall shall be
insulated against thermal shock due to inner tank leakage unless
an analysis shows that this is not needed (e.g. for warmer product
temperatures). Refer to ACI 376 for thermal corner protection.
Materials
9.3.2.1
Materials for bottom and TCP insulation include brittle materials
(cellular glass), materials subject to creep but with closed cells
(polyvinyl chloride, etc.) and, for ring beams, high load bearing
materials. For concrete ring beams, refer to ACI 376.
9.3.2.2
Tests of materials subject to creep are required to establish their
acceptability.
Design
9.3.3.1
Structural design of insulation shall be based on allowable stress
or limit state. For limit state design, follow EN 14620–4 Annex C.
9.3.3.2
For brittle materials the minimum safety factors based on fully
effective interleaving materials are:
For brittle materials the minimum safety factors based on fully
effective interleaving materials are:
a. Normal operation – 3.0 relative to nominal compressive
strength
b. Hydrotest – 2.25 relative to nominal compressive strength
c. OBE Earthquake – 1.25 relative to minimum lower limit
compressive strength
d. SSE Earthquake – 1.0 relative to minimum lower limit
compressive strength
9.3.3.3
For brittle materials with open surface cells an interleaving material
shall be applied to develop the compressive strength of the
material. System tests shall have established the effectiveness of
the material used and the bearing capacity shall be reduced by that
effectiveness. Interleaving materials other than asphalt Type III or
IV shall be tested to include the following:
a. Blocks shall be selected from the same production run.
b. Halves of a minimum of ten blocks shall be used for the control
tests and the other halves for the interleaving material.
c. Control tests shall be per ASTM C240 to duplicate the tests
basis of the material manufacturer.
d. Tests with the interleaving material shall be per ASTM C240
except for sample preparation.
e. Each compressive strength grade shall be tested separately.
Page 44 of 55
9.3.4
9.4
9.3.3.4
For materials subject to creep, the permissible load shall be
established in accordance with EN 14620-4, Section 6.3.2.2.2.
9.3.3.5
The thermal design of the ring beam and any underlying insulation
shall minimize temperatures which are lower under the ring beam
than under the bottom insulation.
Installation
9.3.4.1
Insulation joints shall be staggered with minimum gaps.
9.3.4.2
Insulation shall be installed over a leveling layer of concrete or dry
sand (maximum moisture content 2% by weight) and topped with a
layer of concrete or sand or other material.
9.3.4.3
For brittle materials, an interleaving material shall be applied
between layers, above the top layer and below the bottom layer.
The material shall be butted and not lapped except that the
interleaving material above the top layer may be lapped.
External Wall and Roof Insulation
9.4.1
System Design
External wall and roof insulation systems include rigid insulation covered by a
weatherproofing and vapor barrier or by sealed jacketing that acts as a vapor
barrier. Refer to NFPA 59A Section 7.2.5 for requirements applicable to
external insulation regardless of product stored.
9.4.2
9.5
Design
9.4.2.1
The insulation shall have a weatherproofing or jacketing chosen to
resist site conditions such as marine or polluted atmospheres and
be attached to resist wind.
9.4.2.2
The attachment of the insulation and vapor barrier shall be
designed to accommodate the dimensional changes of the tank.
9.4.2.3
Steel tanks shall be painted or coated prior to insulating.
Internal Wall Insulation
9.5.1
System Design
Internal wall insulation systems include:
a.
b.
c.
Loose fill (e.g. Perlite) in the annular space.
Insulation applied to the outer surface of the inner wall or the inner
surface of the outer wall, or both.
These may also be used between double dome roofs.
Page 45 of 55
9.5.2
Materials
For loose fill insulation, tests shall be conducted during production and
installation of the material (see 9.2.1 and 9.2.2).
9.5.3
9.5.4
9.6
Design and Installation of Loose Fill Insulation
9.5.3.1
For metal inner tanks, a compaction control system typically
consisting of a resilient blanket on the inner tank wall shall be
installed to limit pressure on the inner tank due to filling/emptying
and commissioning/decommissioning. Where this is not installed,
(e.g. liquid oxygen tanks) the inner tank shall be designed for the
uncontrolled Perlite pressure. The purchaser shall specify the
number of commissioning/decommissioning cycles. The
manufacturer shall demonstrate by calculations or tests that the
design pressure is conservative.
9.5.3.2
The method of supporting and attaching any blanket insulation to
prevent failure due to loose fill drag friction shall be submitted by
the manufacturer to the purchaser. The outer layer shall have a
high tensile facing.
9.5.3.3
Loose fill shall be compacted to the specified density by vibration
during installation.
9.5.3.4
A loose fill volume above the annular space extending to the outer
roof shall be provided. This volume shall not be less than 4% of the
loose fill volume in the annular space. A partition shall be provided
on suspended deck designs unless loose fill is also used on the
suspended deck.
9.5.3.5
Loose fill filling nozzles shall be provided so that loose fill may be
added in service. This also applies to loose fill between double
dome roofs.
Design of Insulation Attached to the Walls
9.5.4.1
Insulation shall not disbond on contact with spilled product.
9.5.4.2
The insulation attachment shall be designed to accommodate the
tank movements.
9.5.4.3
For outer steel tanks for double or full containment, an analysis
shall be made to determine the need for insulation of the outer wall
to avoid failure due to thermal shock of leaking product.
Suspended Deck Insulation
9.6.1
If loose fill is used, deck seams must be sealed.
Page 46 of 55
9.6.2
9.7
9.8
For products and atmospheric conditions where condensation can occur in the
space above the deck, the insulation shall be designed so that it cannot be
affected by the condensation.
Penetration and Internal Piping Insulation
9.7.1
Roof nozzle connections with internal cold vapor or liquid process piping shall
be provided with thermal distance pieces where required to hold the roof to
near ambient temperature at the point of penetration. Insulation shall be
provided between the thermal distance pieces and the cold line.
9.7.2
Cold vapor or liquid process piping between the roof and a suspended deck
shall be insulated.
Specifications for Insulation
The following ASTM specifications shall be used in the supply and testing of
insulation.
9.8.1
Cellular Glass




C165 Standard Test Method for Measuring Compressive Properties of
Thermal Insulations
C177 Standard Test Method for Steady-State Heat Flux Measurements
and Thermal Transmission Properties by Means of the Guarded-Hot-Plate
Apparatus
C240 Standard Test Methods of Testing Cellular Glass Insulation Block
C552 Standard Specification for Cellular Glass Insulation
9.8.2. Perlite

C549 Standard Specification for Perlite Loose Fill Insulation
9.8.3. Resilient Glass Fiber Blanket

9.8.4
C764 Standard Specification for Mineral Fiber Loose-Fill Thermal
Insulation
Materials subject to creep (polyvinyl chloride, etc.)
Specifications for these materials shall be proposed by the manufacturer and
approved by the purchaser.
Page 47 of 55
Draft for API Refrigerated Tank Task Group
SECTION 10 - Post Construction Activities
DRAFT NO. D
Rev Date: January 22, 2008
Drafted by: David Nasab & Jack Blanchard
Revision By: Jack Blanchard
Writer’s Comments:
Text in blue indicates changes made since the Oct 26, 2007 draft that was presented at the
Dec 6, 2007 RTTG meeting.
Requirements for testing of pump columns and annular space piping was added.
The entire section was reviewed for “shall” vs. “should”.
10.1
Scope
This section provides requirements and guidance for post construction activities
necessary for the safe startup of storage tank systems covered in this standard.
Activities include pressure testing, purging and cool down.


10.2
Requirements specifically important to concrete structures are provided within ACI
376.
Requirements specifically important to metal structures are provided in API 620.
General
All construction activities, inspections, testing and cleaning (all sand, sludge and
standing water shall be removed) of tank shall be completed. All instrumentation
shall be calibrated and verified prior to final closure of the tank. All electrical systems
including the foundation heating system shall be verified as operational. A drying /
purging and cool down procedure shall be prepared by Tank Contractor for
incorporation into the detailed plant purge and cool down procedure.
10.3
Hydrostatic and Pneumatic Testing
10.3.1 Testing of Primary Liquid and Vapor Containers
Requirements for hydrostatic and pneumatic testing of the primary liquid and
vapor containers are provided within API 620 and ACI 376. Hydrostatic
testing of secondary liquid containers of double and full containment tank
systems is not required unless explicitly specified by the purchaser.
When specified for specific projects, hydrostatic testing of secondary liquid
containers shall include the following:
Page 48 of 55
Hydrostatic testing of the primary container must be completed prior to the
secondary container and shall not be drained prior to filling and emptying the
secondary container.
Verification of primary bottom leak tightness must be made during testing of
the primary liquid container. The bottom insulation system must be protected
from exposure to water during secondary container testing.
Water test height for the secondary container shall, as a minimum, be set at a
height that produces a liquid pressure in the base of the container equivalent
to 1.25 times the pressure produced to contain the full primary container
contents at design liquid level.
Water quality for the secondary container shall meet the water requirements in
API 620.
10.3.2 Pressure Testing of Pump Columns
Pump columns shall be pressure tested, hydrostatically or pneumatically, in
accordance with the standard used for their design (see paragraph 8.2.3).
Pump columns shall be installed prior to hydrostatic testing of the primary
liquid container. Pump column internal pressure testing shall be performed
with the primary liquid container empty, and the pump column shall be empty
when the primary liquid container is hydrostatically tested.
10.3.3 Pressure Testing of Internal Piping
Piping subject to vapor or liquid flow, located between the primary vapor
container and the primary liquid container in double wall tanks shall be
pressure tested as required by API 620 Appendix Q or Appendix R.
10.4
Drying / Purging
Immediately following the hydrostatic test of the tank, residual standing water shall be
removed.
Excessive free water within the insulation system can cause the insulation system to
perform below its design basis and, in the case of cellular glass load bearing
insulation could cause damage to the insulation system. Erection procedures shall
incorporate provisions that eliminate collection of excessive moisture within the
insulation system.
Excessive moisture in the tank atmosphere will be naturally removed from the gas
when the gas temperature drops below the dew point of the gas. Therefore, removal
of most moisture from the gas within the tank will be achieved through the process of
nitrogen and warm gas purges discussed below. The dew point values in Table 10.1
can be used as an indication for when detrimental moisture has been adequately
removed. It is not necessary to lower the dew point below 32oF (0oC). If the
recommended dew point is reached at the end of the nitrogen purge it is not
necessary to take subsequent readings several hours later if the nitrogen purge is
followed by a warm product purge.
Page 49 of 55
A nitrogen purge shall reduce the oxygen level in the tank to a level that will allow the
product to be introduced without creating a combustible gas mixture. The O2 end
point value in Table 10.1 is a value that is considered safe for ethylene per AGA
Principles and Practice – 2001. Percent oxygen in nitrogen gas end points for all
other gasses covered by this standard could be safely set at a higher level but the
dew point values listed will normally be harder to achieve than the 8% O2 level.
A warm product purge to between 80% and 90% product gas normally follows the
nitrogen purge and is completed prior to tank cool down. If liquefied gas is introduced
directly into a nitrogen environment, the initial introduction can cause the temperature
of the liquid to drop below the product design temperature and the design metal
temperature. Material selection and tank design must consider this lower
temperature if a warm product purge is not performed.
An exception to the O2 values in Table 10.1 is ammonia storage. Anhydrous
ammonia storage is susceptible to stress corrosion cracking (SCC). Water additions
have been shown to reduce the SCC process and any free moisture exposed to
ammonia vapor will combine with the ammonia. The percent O2 at time of liquid
accumulation is also important to reduce the SCC process. Therefore, the O2 level
achieved prior to cool down (liquid accumulation) is recommended to be lower than
the value in Table 10.1 and should be as low as practical.
Purge dew point levels recommended for primary concrete containers are found in
ACI 376.
Table 10.1
Recommended Drying and Nitrogen Purging End Points for Steel Tanks
Section
Inner tank and Dome
space
Annular space
(internally insulated double
wall suspended deck
tanks)
Bottom Insulation Space
10.5
Dew Point at 1 atm
O2 Concentration Level (Vol
%)
+23º F (-5º C) Max.
8% Max.
+50º F (+10º C) Max.
8% Max.
No measurement necessary
No measurement necessary
Cool Down
Cool down shall be performed after the tank purge has been completed. A cool down
procedure shall be developed to provide a controlled process. During the initial
introduction of liquid product, it is important to insure that the storage tank cools as
uniformly as possible. Sharp thermal gradients can cause permanent local distortions
and potentially crack growth. The cool down rate for the primary liquid container
should be controlled to an average of 5.4º F/h (3º C/h) but should not exceed 11º F/h
(6º C/h).
Page 50 of 55
For thermal gradients and cool down procedures related to concrete tanks, see ACI
376.
Tanks storing products with an atmospheric boiling point below -60oF shall have a
cool down spray system located near the radial center of the tank to eliminate sharp
thermal gradients on the primary tank bottom. Temperature Elements (TEs) for these
tanks should be located on the inner tank bottom and in a vertical array near or on the
tank shell to monitor the rate of tank cool down.
The cool down can be considered complete when a minimum of 6 inches (150 mm) of
liquid product is maintained in the storage tank. At this point the bottom TEs and TEs
in the first 10 ft of the vertical TE array will be reading approximately product storage
temperature.
When the tank is not cooled down within two weeks after completion of the purging a
small positive pressure should be maintained to prevent ingress of oxygen and
moisture. The purge end point for the annular space, as measured at the end of the
nitrogen purge, should also be reduced 5oC to stabilize the moisture levels in the
tank.
Page 51 of 55
Draft for API Refrigerated Tank Task Group
Appendix A – Properties of Gasses
DRAFT NO. D
Date: February 6, 2008
Drafted by: Jack Blanchard & David Nasab
Chairman Comments:
Changes since August, 2007 ballot are:
1) Additional properties added; critical point temperature, critical point pressure, gas/liquid ratio.
2) Data is slightly different being from a different source. Source of properties data now named.
3) SI units table added
4) Introductory text added
Page 52 of 55
This informational appendix provides useful reference data. Sourses of the values given are as follows. LFL / UFL Flammability Limits are
from NFPA 497. All other values are verified by HYSYS, (Process modeling analysis computer program by AspenTech). Unless otherwise
noted, physical property values have been calculated based on the following conditions: Temperature = 70oF and Pressure(atmospheric)= 14.7 psi.
Note that all values are for pure gasses. Actual stored liquid gases may contain a combination of several gasses. Design values to be used
on specific tank designs may be somewhat different and shall be provided according to the responsibilities outlined in Section 4.
Table A-1a Physical Properties of Gases (SI)
GAS
Chemical
Formula
Air
MOLECULAR
WEIGHT
SPECIFIC HEAT
BOILING (Cp) (Mass Heat
POINT
Capacity)
AT 1 ATM
at conditions
defined above
(oC)
(kJ/kg oC)
28.95
CRITICAL
POINT
TEMP
(oC)
CRITICAL
POINT
PRESS
(kPa abs)
0.99
VAPOR
DENSITY
at
conditions
defined
above
(kg/m3)
GAS/LIQUID
VAPOR
LIQUID
DENSITY at (1 DENSITY at (1 RATIO (15.6oC
ATM) BOILING ATM) BOILING
1 ATM /
POINT
POINT
BOILING
(kg/m3)
(kg/m3)
POINT)
HEAT OF
VAPORIZATION
at (1 ATM)
BOILING PT
(kJ/kg)
FLAMMABILITY
LIMITS
(LFL / UFL)
1.22
Argon
Ar
39.95
-185.83
0.52
-122.5
4895.2
1.69
5.75
1389.79
823
162.12
-
Nitrogen
N2
28.01
-195.72
1.04
-147.0
3399.8
1.18
4.60
804.62
680
199.45
-
Oxygen
O2
32.00
-183.00
0.91
-118.6
5042.8
1.35
4.46
1145.50
847
213.88
Methane
CH4
16.04
-161.61
2.23
-82.6
4594.6
0.68
1.81
423.54
624
509.16
5.0 / 15.0
Ethylene
C2 H4
28.05
-103.67
1.55
9.2
5040.0
1.19
2.08
566.58
475
486.37
2.7 / 36
Ethane
C2 H6
30.07
-88.61
1.73
32.2
4871.1
1.28
2.05
545.44
426
490.09
3 / 12.5
Propylene
C3 H6
42.08
-47.24
1.49
91.8
4609.8
1.81
2.34
608.55
337
443.34
2.0 / 11.1
Propane
Anhydrous
Ammonia
C3 H8
44.10
-42.03
1.49
96.7
4247.1
1.81
2.40
581.32
322
428.45
2.0 / 9.5
NH3
17.03
-33.41
2.10
132.3
11348.7
0.73
0.88
673.59
928
1376.76
15 / 28
Iso-butane
C4H10
58.12
-11.54
1.66
134.7
3639.7
2.53
2.81
593.65
235
367.97
1.8 / 8.4
1-Butene
C4 H8
56.11
-6.32
1.50
146.4
4023.1
2.44
2.65
622.97
256
398.68
1.6 / 10.0
1,3-Butadiene
C4 H6
54.09
-4.40
1.49
151.7
4326.4
2.35
2.54
650.20
277
417.75
2.0 / 12.0
n-Butane
C4H10
58.12
-0.27
1.66
152.0
3796.2
2.53
2.69
601.50
238
390.54
1.9 / 8.5
Page 53 of 55
Table A-1b Physical Properties of Gases (US Customary Units)
GAS
Chemical
Formula
Air
MOLECULAR
WEIGHT
SPECIFIC HEAT
(Cp) (Mass Heat
BOILING PT
Capacity)
at 1 ATM
at conditions
o
( F)
defined above
(BTU/lb oF)
28.95
CRITICAL
POINT
TEMP
(oF)
CRITICAL
POINT
PRESS
(psi abs)
VAPOR
VAPOR
LIQUID
GAS/LIQUID
DENSITY
DENSITY at (1 DENSITY at (1
at conditions
RATIO (60oF 1
ATM) BOILING ATM) BOILING
defined
ATM / BOILING
POINT
POINT
above
POINT)
(lb/cu ft)
(lb/cu ft)
(lb/cu ft)
0.24
HEAT OF
VAPORIZATION
at (1 ATM)
BOILING PT
(BTU / lb)
FLAMMABILITY
LIMITS
(LFL / UFL)
0.08
Argon
Ar
39.95
-302.50
0.12
-188.5
710.0
0.11
0.36
86.76
823
69.70
-
Nitrogen
N2
28.01
-320.30
0.25
-232.5
493.1
0.07
0.29
50.23
680
85.75
-
Oxygen
O2
32.00
-297.40
0.22
-181.4
731.4
0.08
0.28
71.51
847
91.95
Methane
CH4
16.04
-258.90
0.53
-116.7
666.4
0.04
0.11
26.44
624
218.90
5.0 / 15.0
Ethylene
C2 H4
28.05
-154.60
0.37
48.5
731.0
0.07
0.13
35.37
475
209.10
2.7 / 36
Ethane
C2 H6
30.07
-127.50
0.41
89.9
706.5
0.08
0.13
34.05
426
210.70
3 / 12.5
Propylene
C3 H6
42.08
-53.03
0.36
197.2
668.6
0.11
0.15
37.99
337
190.60
2.0 / 11.1
Propane
Anhydrous
Ammonia
C3 H8
44.10
-43.66
0.36
206.1
616.0
0.11
0.15
36.29
322
184.20
2.0 / 9.5
NH3
17.03
-28.13
0.50
270.2
1646.0
0.05
0.05
42.05
928
591.90
15 / 28
Iso-butane
C4H10
58.12
11.23
0.40
274.5
527.9
0.16
0.18
37.06
235
158.20
1.8 / 8.4
1-Butene
C4 H8
56.11
20.62
0.36
295.5
583.5
0.15
0.17
38.89
256
171.40
1.6 / 10.0
1,3-Butadiene
C4 H6
54.09
24.08
0.35
305.0
627.5
0.15
0.16
40.59
277
179.60
2.0 / 12.0
n-Butane
C4H10
58.12
31.51
0.40
305.6
550.6
0.16
0.17
37.55
238
167.90
1.9 / 8.5
Page 54 of 55
APPENDIX B
Foundation Settlement (drafted by Jack Blanchard)
Depending on the loads, temperature, time and other design parameters, settlement effects
can result in unacceptable stress levels that are either positive or negative for the structure
design.
Tank settlement patterns and resultant tank distortions can be very complex and
unpredictable. Important factors that can affect how a tank reacts to settlement include
heterogeneous soils (both vertically and horizontally), variable as-built distortions, short vs.
long term, and sensitivity of structural details to movement.
Predicted settlements shall be determined as part of the site specific geotechnical study.
Soil Improvement, as determined necessary by design of the tank system may be applied to
reduce the predicted settlements.
Settlement can be separated into four specific types: uniform settlement, global tilt,
differential center-to-edge settlement and differential circumferential settlement. Values
provided below are based on experience and intended for guidance. Variations from these
values are acceptable if accounted for in the design of the tank and foundation system.
a. Uniform settlement: The amount of acceptable uniform edge settlement is
dependent upon the connections to the tank.
b. Global Tilt: Global tilt (also addressed as planar tilt) is associated with rigid body
rotation of the superstructure caused by non-uniform soil across the width of the
structure. While large tanks may be able to accommodate significant uniform tilt
without damage, other components usually require lower values of tilt.
Predicted global tilt, measured in inches, of a flat bottom tank shell should be
limited to 5 times the tank diameter divided by the tank height.
c. Differential Center-to-Edge Settlement: Liquefied gas tanks are constructed
with self supporting roofs. Differential settlement between the center and the
edge does not affect the roof. While bottom plate can accommodate significant
settlement, tank internals supported by the bottom and the bottom insulation
system inherent in liquefied gas tanks cannot. Significant short or long term
settlement of the bottom can crack or damage the bottom insulation system which
would seriously increase the heat leak of the structure, potentially causing
freezing of the soil under the tank.
Predicted differential settlement between the edge of the tank and the center
should be limited to the radius of the tank divided by 240.
d. Differential Circumferential Settlement: Irregular settlement of soils around the
periphery of a tank can cause out-of-roundness and localized distortions and
buckles in a tank. These can affect the stability or the performance of the tank.
Predicted differential settlement around the periphery of the tank should be limited
to 1:1000 (i.e. 3/8 inch in an arc radius of 30 feet).
Page 55 of 55